1. Filed of the Invention
The present invention relates to a receiver circuit for receiving an analog modulated signal as well as a digital modulated signal.
2. Description of the Related Art
Cellular communication systems that are currently in service as mobile radio communication systems are required to increase system capacity as the number of subscribers increases. To meet this need, a switch from the conventional analog cellular communication systems to digital cellular communication systems is taking place in various countries in North America, Europe, and in Japan. In North America, both analog and digital cellular communication systems employ the same frequency band, and cellular terminals must therefore be dual-mode devices capable of connecting with analog and digital cellular communication systems.
FIG. 1 of the accompanying drawings shows a receiver circuit of a conventional dual-mode device. The receiver circuit comprises a high frequency amplifier 1, a bandpass filter 2, a down conversion block 16, intermediate frequency bandpass filters 8, 10, an intermediate frequency signal linear amplifier 9, a limiter amplifier 11, an analog demodulator 12, an automatic-gain-control amplifier 19, a digital demodulator 14, and a received signal intensity detector 15. The received signal intensity detector 15 outputs received signal intensity information S.sub.RSS. The down conversion block 16 comprises mixers 3, 6, local oscillators 4, 7, and a bandpass filter 5.
The receiver circuit shown in FIG. 1 operates as follows:
A digital modulated signal that has been received is amplified by the high frequency amplifier 1 and then limited to a certain received frequency band by the bandpass filter 2. The digital modulated signal is thereafter converted into an intermediate frequency signal by the down conversion block 16. The digital modulated intermediate frequency signal from the down conversion block 16 is passed through the intermediate frequency bandpass filter 8 and applied to the intermediate frequency signal linear amplifier 9, which linearly amplifies the applied signal. The amplified signal from the intermediate frequency signal linear amplifier 9 is again limited to a frequency band by the intermediate frequency bandpass filter 10. The digital modulated intermediate frequency signal from the intermediate frequency bandpass filter 10 is then linearly amplified by the automatic gain control amplifier 19. The digital demodulator 14 produces I and Q signals from the amplified signal from the automatic gain control amplifier 19. The I and Q signals pass through respective root Nyquist filters (not shown), which output orthogonally demodulated signals. An analog modulated signal that has been received is processed into an intermediate frequency signal by the same circuit components as the digital modulated signal up to the intermediate frequency bandpass filter 10. The intermediate frequency signal produced from the analog modulated signal is then limited to a certain amplitude and amplified by the limiter amplifier 11 and demodulated into an analog demodulated signal by the analog demodulator 12.
A conventional receiver circuit capable of receiving both digital and analog modulated signals is composed of many parts and is complex in circuit arrangement because for digital signal demodulation it requires the automatic gain control amplifier for linearly amplifying the intermediate frequency signal and the bandpass filter having characteristics approximating root Nyquist characteristics to limit the frequency bands of the I and Q signals, and for analog signal demodulation, it also requires the limiter amplifier for limiting and amplifying the intermediate frequency signal.